1. Field of the Invention
The present invention relates to a pressure sensor suitable for use downhole in oil, gas, geothermal and other wells, at the wellhead, in industrial applications, for portable calibration devices and in laboratory applications. More specifically, by way of example and not limitation, the invention in its prefected embodiment relates to a piezoelectrically-driven quartz crystal resonator pressure sensor configured for enhanced sensitivity and accuracy.
2. State of the Art
The type of quartz crystal pressure transducer assembly in which sensors as disclosed herein may preferably be employed includes a first pressure sensitive quartz crystal resonator, a second temperature sensitive quartz crystal resonator, a third reference frequency quartz crystal resonator, and supposing electronics. For convenience, the terms "crystal" and "resonator" may be used interchangeably herein in referencing a resonating quartz crystal element.
In a transducer assembly of the referenced type, the first crystal changes frequency in response to changes in applied external pressure and temperature, while the output frequency of the second crystal is used to temperature compensate temperature-induced frequency excursions in the first and third crystals. The third crystal generates a reference signal, which is only slightly temperature dependent, against or relative to which the temperature and pressure-induced frequency changes in the first crystal and the temperature-induced frequency changes of the second crystal can be compared. Means for comparison as known in the art include frequency mixing and/or using the reference frequency to count the signals from the other two crystals. The first resonator is exposed via a fluid interface to the external pressure sought to be measured, and all three resonators are preferably thermally coupled to the fluid to provide a rapid thermal response time. The transducer (crystals plus electronics, the latter disposed in a pressure housing) is calibrated as a complete unit over the intended pressure and temperature range so that all temperature and pressure related effects can be compensated for in the resulting calibration curve-fit coefficients. Exemplary patents for transducers using three crystal resonators, each assigned a function as described above, are U.S. Pat. No. 3,355,949 to Elwood et al., U.S. Pat. No. 4,802,370 to EerNisse et al. and U.S. Pat. No. 5,231,880 to Ward et al.
The first crystal, or pressure sensor crystal, employed in pressure transducer assemblies of the prior art, has been commonly configured to include a disc-shaped resonator element incorporated in a tubular cylindrical housing assembly, the ends of the housing assembly being closed. The cylindrical housing assembly, when subjected to exterior pressure of a fluid to be monitored, elastically deforms and thus causes the frequency of the resonator element to shift, the frequency output thus being indicative of the pressure. As noted above, the frequency output may then be preferably temperature-compensated, as known in the art. Exemplary pressure sensor crystal configurations are disclosed in U.S. Pat. No. 3,561,832 to Karrer et al., U.S. Pat. No. 3,617,780 to Benjaminson et al., U.S. Pat. No. 4,550,610 to EerNisse, U.S. Pat. No. 4,660,420 to EerNisse, U.S. Pat. No. 4,754,646 to EerNisse et al., U.S. Pat. No. 4,802,370 to EerNisse et al., U.S. Pat. No. 5,221,873 to Totty et al., and in EerNisse, "Quartz Resonator Pressure Gauge: Design and Fabrication Technology," Sandia Laboratories Report No. SAND78-2264, (1978).
U.S. Pat. No. 4,660,420 to EerNisse recognizes the desirability of selecting a pressure crystal with a crystal cut having substantial independence from temperature-induced frequency changes over the intended range of temperatures, as well as a relatively large scale factor, i.e., greater frequency sensitivity to pressure changes in the range to be measured. For the pressure and temperature ranges experienced in oil and gas wells, an AT-cut quartz crystal is disclosed in EerNisse '420 to possess these attributes.
Yet another EerNisse patent, U.S. Pat. No. 4,754,646, discloses the use of an integral housing and resonator section preferably formed from AT-cut, BT-cut or rotated X-cut quartz, but does not distinguish the performance characteristics of any of the various cuts, or recommend a particular cut. Rather, EerNisse '646 seeks to reduce resonator resistance and pressure hysteresis via particular physical configurations of the resonator and its area of joinder to the outer cylindrical shell.
While prior art devices as referenced above have attempted to address various deficiencies in the generic quartz resonator sensor design, those of ordinary skill in the art have failed to recognize that pressure sensitivity of such sensors may be greatly enhanced and inaccuracy reduced over a broad pressure range through certain relatively straightforward modifications to physical parameters of the sensor configuration. It has also gone unrecognized that the upper limit of the range of such enhanced-sensitivity sensors may be readily altered through other, equally straightforward modifications to the sensor, so that a sensor may offer such enhanced sensitivity and accuracy over a much larger range than prior art sensors.